From what I understand of the original Forge definition, a "heartbreaker" is basically a type of ripoff or a homebrew campaign that the GM decided to spin off into a published book without publishing it under the OGL. The part that breaks the reviewer's heart is that it has just enough originality and creativity in parts that you think the creator could do better. I generally use it that way myself.
A lot of games get called heartbreakers when they really aren't, at least according to this definition. For example, a lot of 90s urban fantasy games like
Nephilim,
Nightlife,
Immortal: The Invisible War,
Fireborn,
Everlasting,
C.J. Carella's WitchCraft, etc. are dismissed as
World of Darkness heartbreakers when they're really nothing of the sort. They're in the same genre, sure, but they're not obvious derivatives. They do plenty to make themselves stand out.
However, in recent years I have noticed that there is a stream of what I think I can genuinely describe as heartbreakers of
Vampire: The Masquerade or
C.J. Carella's WitchCraft.
Blood Junkies,
The Vampyre Hack,
Vampire: Undeath,
Wine Dark Nights, etc. are all games I've come across that are clearly very derivative, even outright admitting so in their introductions, even though they don't have to be. They make
Vampire: The Requiem, which itself has been criticized as an official heartbreaker of its predecessor, and
Everlasting: The Book of the Unliving, which was written by a WW freelancer as a clear response, look original by comparison. The heartbreaker part comes from my observation that they often introduce creative original ideas, like vampires scrying memories from imbibing the blood of their victims, making thematic vulnerabilities stem from a vampire's powers instead of their bloodline, making a vampire's power level a function of their age and experience instead of how many steps they're removed from their bloodline's progenitor, or having a vampire's internal struggle between humanity and vampirism operate like a lightside/darkside mechanic instead of a punitive "you get schizophrenia for stealing a candybar."
Have you ever heard of that joke about how unrelated crustaceans keep evolving crab-like body plans? I've noticed something similar with
WitchCraft. It's been canceled for decades so I doubt anyone making games now has read it, but I noticed that they keep independently recreating many of its innovations, such as having multiple ideologically driven factions, multiple magical races coexisting, multiple magic systems, and kitchen sink cosmologies. I've seen this used to some degree in
Vampire: The Requiem,
Hunter: The Vigil,
Nephilim: Revelation (French only, sorry),
Liminal, and so on. The heartbreaker part comes in because none of these actually go as far as
WitchCraft itself in being a broad flexible kitchen sink setting. They are much more narrow and idiosyncratic by comparison.
This seems like its only going to become more common. There are two very blatant
Vampire heartbreakers currently in development,
Nightborn and
Harrowed World. They're video game tie-ins and according to their teasers, they have almost identical premises.
Nightborn has 10 classes and 3 joinable factions fighting for domination a la
Vampire: The Masquerade.
Harrowed World has 8 classes (some of which are duplications, there's this whole metaphysical thing going on) and 6 joinable factions with shifting alliances a la
Vampire: The Requiem. Likewise, recent leaks suggest that Chaosium will release a second edition of
Nephilim that has had its lore stripped out in favor of a
WitchCraft-esque structure of five factions coexisting.
As you can probably guess from the fact that I call all of these heartbreakers, I'm not happy with this state of affairs. I feel like games are becoming more diluted and homogenized without any improvement. I really don't like
World of Darkness, even long before the fifth edition controversies, due to its many self-referential self-iterative idiosyncrasies and I vastly prefer
WitchCraft for its broad flexible setting that reflects how the urban fantasy genre functions in prose fiction. I did like what
Chronicles of Darkness tried to do, even if I otherwise don't agree with a lot of creative decisions, but being constantly cyberbullied for liking it drove me away from the hobby. I liked the concept behind
Nephilim because it made lore personally relevant to PCs where other games introduce lore as irrelevant self-aggrandizing microfiction that I despise, so stripping that out in favor of telling GMs to make stuff up instead removes everything that attracted me in the first place. The changes don't make it a remotely suitable substitute for
WitchCraft either because it still has tons of weirdo baggage like the PCs being thetans who possess human bodies and steal babies' souls.